1. Field of the Invention
The present invention relates to an illumination optical system, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate devices such as a semiconductor device and liquid crystal panel by using photolithography. The projection exposure apparatus includes an illumination optical system for illuminating a reticle (mask) on which a circuit pattern is formed, and a projection optical system for projecting the pattern of the reticle onto a substrate such as a wafer, and transfers the pattern of the reticle onto the substrate.
Optical systems such as an illumination optical system and projection optical system generally have a feature that points spaced apart equidistantly from the optical axis of the optical system, that is, points at the same image height exhibit the same optical characteristics from the viewpoint of design. Japanese Patent Laid-Open No. 60-232552 proposes an exposure apparatus including a projection optical system in which that feature is applied to two concentric mirror systems (a concave mirror and convex mirror), and which projects the pattern of a reticle onto a wafer using an off-axis annular imaging region (effective image region).
FIGS. 18A to 18F are views for explaining a conventional exposure apparatus 1000 disclosed in Japanese Patent Laid-Open No. 60-232552. The imaging region of a projection optical system 1300 in the exposure apparatus 1000 is an arcuated region A formed to have a predetermined radius using an optical axis O as the center, as shown in FIG. 18B. An illumination optical system 1200 needs to illuminate the imaging region of the projection optical system 1300 with a high illuminance.
In the illumination optical system 1200, an elliptic mirror 1210 converges a light beam emitted by a mercury lamp 1100 at a focal point. An optical integrator 1220 set near the focal point uniformly illuminates (Kohler-illuminates) the back-side focal plane of a first condenser lens 1230 as a first surface to be illuminated FIP. The light beam having propagated through the first surface to be illuminated FIP illuminates a reticle via a second condenser lens 1240.
Note that the optical integrator 1220 includes a cylindrical lens array having an optical refractive power, as shown in FIG. 18C. The optical refractive power differs between two orthogonal directions, as shown in FIGS. 18D and 18E. For this reason, a rectangular illumination region IA is formed on the first surface to be illuminated FIP, as shown in FIG. 18F. Because a slit having an arcuated opening OP is set immediately beneath the first surface to be illuminated FIP, the light beam having passed through the opening OP of the slit forms an arcuated illumination region on the reticle via the second condenser lens 1240. With this arrangement, the illumination optical system 1200 can illuminate only the imaging region (arcuated region) of the projection optical system 1300.
Unfortunately, because the conventional exposure apparatus uses an arcuated illumination region extracted from a rectangular illumination region, as shown in FIG. 18F, the illumination efficiency decreases in proportion to the area ratio between the rectangular illumination region and the arcuated illumination region. This results in a decrease in the integrated illuminance (exposure amount) in exposure, leading to a decrease in the throughput (the number of wafers processed per unit time) serving as one important parameter in the exposure apparatus.